A variety of different molded parts can be produced by reaction injection molding ("RIM") process. This process involves filling a closed mold with highly reactive, liquid starting components within a very short time, generally by using high out-put, high pressure dosing apparatus after the components have been mixed. The RIM process has become an important process for the production of external automotive body parts and other types of molded products. The RIM process involves the intimate mixing of a polyisocyanate component and an isocyanate-reactive component followed by the injection of this mixture into a mold for subsequent rapid curing. The polyisocyanate component is typically based on a liquid polyisocyanate. The isocyanate-reactive component contains a high molecular weight isocyanate-reactive component, typically a polyol and/or an amine polyether, and usually contains a chain extender containing amino and/or hydroxyl groups. U.S. Pat. No. 4,218,543 describes a RIM process currently being commercially used on a large scale. U.S. Pat. Nos. 4,433,067, 4,444,910, 4,530,941, 4,774,263, and 4,774,264 describe reaction injection molding processes for the production of polyurethane(urea) elastomers.
In spite of the many advantages of the RIM process, there is a continual search for faster reactive systems, particularly for use in mass production industries, such as the automotive industry. However, such active systems would gel so quickly upon mixing of the components that the final products produced would have relatively high densities (i.e., 1.12 grams per cubic centimeter) since conventional organic blowing agents do not volatilize quickly enough to have any blowing effect. One suggestion, which has met with some success, has been to include in one or more of the components air and/or nitrogen under pressure. The use of air and/or nitrogen in polyurethane systems is, of course, known, as are the many and varied techniques for providing such dissolved air and/or nitrogen. For example, air and/or nitrogen has been introduced directly into the mixing chamber and mixed simultaneously with the reactive mixture. Additionally, the air and/or nitrogen has been whipped into one or more of the components. The creamy mixture formed is then metered by means of a pump to a final mixing chamber where it is mixed with the other reactive components. When the metering pump discharges at a sufficiently high pressure, the quantity of gas which is initially dissolved and/or dispersed in the starting material, which is fed to the metering pump, dissolves at the higher pressure in a very short period of time. The liquid fed to the mixhead then contains gas in the dissolved state. Upon being fed to the mixhead, dissolution takes place in a very short time. In general, it is preferred that the gas be dissolved in one or more of the components. Other techniques for dissolving gases are also known and are described in U.S. Pat. Nos. 4,089,206 and 4,050,896.
Although the use of such dissolved air and/or nitrogen has met with some success with highly active systems, it has been found that the resultant molded part, while of reduced density (e.g., from 0.99 to 1.09 grams per cubic centimeter), will have varied densities throughout the molded part. As noted above, air and/or nitrogen is effectively dissolved under pressure in one or more of the components. It has been observed that when this pressure is relieved (e.g., upon passage of the components through the mixhead and into the mold), the air and/or nitrogen does not immediately pass from the dissolved state to the dispersed state. It is believed that a state of super saturation exists in liquid reacting system containing the dissolved gas for some finite period of time. For highly reactive systems, this delay in passage from the dissolved to the dispersed state is sufficiently long so that gelation occurs in the mold before proper blowing.
Expanded microspheres consisting of a synthetic thermoplastic resin shell that encapsulates a liquid blowing agent are known. See, e.g., U.S. Pat. Nos. 4,829,094, 4,843,104, and 4,902,722. Such microspheres have been suggested for use in plastics, coatings and adhesives, and are described as having the ability to reduce density, to lower volume costs, to improve impact resistance, and to reduce shrinkage (see "Dualite" product information bulletin). In addition, such microspheres have been described as useful in low density rapid setting polyurethanes (see U. S. Pat. No. 4,038,238) and in non-polyurethane-based reaction injection molded polymers (see, e.g., U.S. Pat. No. 4,959,395). Finally unexpanded microspheres have been described for use in polyurethane RIM (see Japanese Patent Publication 60-244511).